Sahih Bukhari

Narrated Ibn `Abbas:The word 'Al-Kauthar' means the abundant good which Allah gave to him (the Prophet Muhammad (ﷺ)). Abu Bishr said: I said to Sa`id, ""Some people claim that it (Al-Kauthar) is a river in Paradise."" Sa`id replied, ""The river which is in Paradise is one item of that good which Allah has bestowed upon him (Muhammad)